A compact acoustic transducer, that is, an earphone has been widely used which converts electric signals output from a reproduction apparatus and a receiver into an acoustic signal by a speaker near an ear or an eardrum. This kind of the sound reproduction apparatus generates a sound so that a listener who wears the reproduction apparatus can listen to the sound. Therefore, the sound reproduction apparatus is used in various environments.
Many of currently popular earphones have shapes to be inserted into the ear of the listener. For example, an inner ear type earphone has a shape to be hooked on an auricle of the listener. Also, a canal type earphone has a shape used by being deeply inserted into a hole of the ear (ear canal). Also, the canal type earphone often has a closed structure and has relatively good sound insulating performance. Therefore, with the canal type earphone, there is a merit that the listener can enjoy music in a slightly noisy place.
In general, the canal type earphone includes a speaker unit for converting an electric signal into an acoustic signal and a substantially cylindrical housing which is also used as an acoustic tube as basic components, and the speaker unit is attached to one end (outer side of ear canal) of the housing. In the housing, a radiation exit is provided which radiates air vibration generated by the speaker unit to the ear canal and transmits the air vibration to an eardrum. Also, an earpiece (detachable component) is attached to another end (part inserted into ear canal) of the housing. The earpiece normally has a shape which matches the ear canal when the listener wears the earpiece.
For example, a canal type earphone device has been proposed which can house a housing in a cavum conchae and can arrange an acoustic tube to an ear canal entrance by obliquely arranging the acoustic tube from a position shifted from the center of the housing (for example, refer to Patent Document 1).
It is necessary for the canal type earphone, at a minimum, to include the speaker unit and the housing to which the speaker is attached and which contains the speaker and obtains an acoustic characteristic at the same time. In other words, the canal type earphone needs to have a weight of the speaker unit and the housing and needs to have a volume of the housing. This causes a feeling of foreign materials to remain when the user wears the earphone on the ear.
Also, with the traditional earphone device (including inner ear type and canal type), basically, a sound generated by the speaker unit passes through the earphone housing and the ear canal and reaches the eardrum. Then, the sound vibrates the eardrum so that the user listens to the sound.
Also, the sound which has reached the eardrum is reflected by the eardrum and travels in the reverse direction in the ear canal to get outside. However, many of the traditional earphone devices have a configuration in which the earphone housing is worn on a place near the auricle so as to cover the ear canal entrance. Therefore, the sound (reflected sound from eardrum) which is getting outside of the ear canal is reflected by the earphone housing and the speaker unit in the housing and enters the ear canal toward the eardrum again.
In a word, due to the repetition of the reflection, when the traditional earphone device is used, two kinds of sounds, i.e., the sound which has been directly entered from the speaker unit to the eardrum and the sound which has been reflected by the eardrum once and reflected by the earphone housing and the like again, are listened. When a time interval between the directly entered sound and the reflected sound is equal to or shorter than several hundred microseconds, the time interval is turned into localization phenomenon and a feeling of pressure in auditory sense and acts on the user. Therefore, this inhibits listening to the sound as an excellent reproduced sound.
An earphone device for using an acoustic tube having a non-reflection end to prevent the sound reflected by the eardrum from being re-reflected by the earphone housing and the like has been known. The inside of this kind of acoustic tube is basically the same as a free space. A traveling wave of the sound wave (audible sound) generated from the speaker provided at one end of the acoustic tube is propagated, and a reflected wave is not generated. Therefore, the above earphone device becomes a non-reflection earphone.
For example, an acoustic reproduction apparatus has been proposed which includes an acoustic tube formed to have an inner diameter substantially the same as that of an ear canal and a speaker unit attached in a state where a sound emitting surface is faced to an inner wall surface of the acoustic tube (refer to Patent Document 2). Also, in the acoustic reproduction apparatus, an internal circumference area including the sound emitting surface of the speaker unit of the acoustic tube is substantially the same as an internal circumference area which does not include the sound emitting surface of the speaker unit of the acoustic tube. Also, one end of the acoustic tube is a part fitted into an auricle, and the other end is a sound non-reflection end. According to the acoustic reproduction apparatus, the voice emitted from the speaker unit reaches the eardrum through the acoustic tube, and the voice is reflected to the non-reflection side of the acoustic tube after being listened by the eardrum. Therefore, the voice is not reflected toward the eardrum side and is not listened again.
However, although the acoustic reproduction apparatus for using the acoustic tube can remove the effect of the reflected sound, the acoustic reproduction apparatus needs to include the acoustic tube in addition to the speaker unit. Also, the weight and the volume of the speaker and the housing cause the feeling of foreign materials to remain when the user wears the acoustic reproduction apparatus on the ear.